262 EDWIN S. GOODRICH. 



ing the optic centres, corresponds to the first segment in 

 Peripatus, The second or deutocerebrum, supplying the an- 

 tennae, corresponds to the mandibular segment; whilst the 

 third, or tritocerebrura, represents the segment in Peripatus 

 which supplies the oral papillas. 



It would appear, then, that in the Hexapoda the prosto- 

 mium and archicerebrura have not been plainly distinguished ;i 

 that the large ophthalmic segment represents the primitive 

 peristomium, or first metamere, which has shifted in front of 

 the mouth together with the antennary or second metamere. 



The Arachnida (figs. 9 and 10). 



In the Arachnids the head appears to be formed of two 

 segments — the anterior represented by the procephalic lobes 

 in the embryo, and the posterior by the segment bearing the 

 chelicerffi. The syncerebrum is formed by ganglionic masses 

 from these two regions. 



Concerning the metameric nature of the cheliceral segment 

 there can be no doubt. It is primitively distinctly post-oral 

 in position, and contains a pair of mesoblastic somites with 

 coelomic cavities extending into the appendages. These are 

 innervated by a pair of primitively ventral post-oral ganglia, 

 which subsequently move forward and dorsally to form the 

 second and posterior segment of the syncerebrum. 



The procephalic lobes, on the other hand, offer almost the 

 same difficulties of interpretation as in Hexapoda. As a rule, 

 they are separated from the cheliceral segment only after the 

 appearance of several more posterior segments. No distinct 

 appendages are produced. This large ''procephalic" region, 



' Mr. Wheeler says "it is extremely improbable that so highly important a 

 structure as the Annelid brain should have completely disappeared in the 

 Arthropods" (18). This no doubt is quite true; yet it must not be taken for 

 granted that in the remote Annelidan ancestor of the Arthropoda the brain was 

 as important and fully differentiated an organ as in certain modern Polychgetes ; 

 and anyhow it must be admitted as a fact that some of the functions of such 

 an archicerebrum have come to be shared, if not usurped, by the ganglia of 

 posterior metameres. 



